Add support for style block with more than 3 fence character

[gagahpangeran]

I noticed this comment while discussing something is #osu-wiki discord.

osu-web/app/Libraries/Markdown/StyleBlock/Parser.php

Lines 42 to 46 in b6cd03f

	// TODO: closing block length is supposed to match opening block length which can be > 3;
	// what's supposed to happen when they don't match currently is undefined.
	if ($currentLine === ':::') {
	return BlockContinue::finished();
	}

Now style block can use more than 3 fence character and also automatically can support nested style block by using different length of fence chars.